**6.3 Volatile solids**

The solids concentration of the influent into the biodigester affects the rate of fermentation (Marchaim, 1992). In a reported experiment conducted in China, which is mostly located in the temperate latitudes, the optimum concentration of solids was considered to be 6% in summer but between 10 and 20% in winter and spring. When temperatures are low and materials take longer to decompose; it is better to have a higher total solids concentration, although this might cause a problem with impeded flows through the digesters (San Thy et al., 2003). The loading rate is defined as the amount of volatile solids (fermentable solids) per unit of active biodigester volume per day. Typical values of loading rates are between 0.2 and 2 kg VS/m3/day. This assumes that total solids (TS) are 17% of the fresh weight of the manure and that the volatile solids content (VS) is 77% (Fulford, 1988). The methane content of the gas can indicate overloading but it is more difficult to measure unless the right equipment is available. If the digester is being overloaded, the gas production will rise up initially and then fall after a while when inhibition occurs. The CH4 content of the gas will fall while the CO2 content will rise, because CO2 is not used by the hydrogen consuming bacteria or because the methanogenic bacteria are inhibited.

The feedstock concentration of volatile solids (VS), the detention time, and the operating temperature are the major design factors, which determine the maximum total daily methane production (Hill, 1982). In a study by Vetter et al., (1990), daily biogas production was determined to be directly proportional to the volatile solids loading rate, given that other factors such as digester temperature and pH stayed relatively the same. Hill (1982) found that maximum VS reduction based on developed kinetic data was 75, 56, 30 and 62 percent for pig, beef, dairy, and poultry waste respectively. No significant increase in VS destruction will occur at temperature greater than approximately 450C.
